The present invention relates to a method and apparatus for the formation of can bodies. Various types of ductile metal cans are used to provide packaging for a wide variety of foods, beverages and others products, the package being hermetically sealed and possibly under natural or created pressure conditions, or under vacuum. One popular can, offered in different sizes, is the so-called beer/beverage can having a unitary drawn can body to which an easy-opening end is attached after filling. Other cans, often of the wide-mouth type, are used to package cheese spreads, nuts, and other food products which may be only hermetically sealed, or may be vacuum packed in some instances or packed with an inert gas under pressure in other instances. In aggregate, the market demand for all these various types of unit-body cans amounts to billions of units.
By way of a striking example, those cans used for containing beer and soft drinks are required to withstand internal pressure, rough handling, and substantial temperature differences, yet maintain a complete hermetic seal to protect the contents of the can. Cans of this type alone are used in very large volume, billions of cans per year, and at present the metals most used for fabricating such cans are aluminum and thin coated steel due to their comparative inexpensiveness and workability.
The typical modern can consists of a unitary deep drawn body that includes both a cylindrical side wall (sometimes outwardly and upwardly tapered in the case of food cans) and an integral bottom wall closing one end of the can. Usually, the beverage can body has a necked inward throat at its top which terminates in an outwardly extending body curl. An end for the can is provided with an end curl which is designed to interact with the body curl in seaming apparatus to attach the end to the can body after filling of the can to provide the requisite hermetic seal.
Both the can bodies and the can ends are formed from sheet or coil stock material. In view of the large quantities of cans and ends that are manufactured, it is economically very desirable to form the can bodies and ends from as thin a stock material as possible while retaining the necessary pressure-resistant strength therein. At the same time, it is also desirable to minimize the amount of waste of the stock material generated during the fabrication process.
A number of methods are known for fabricating the can bodies. In one method, known as the "draw and iron" method, a circular blank is formed from the stock material. The blank is then drawn in a ram press to form a cup-shaped receptacle having a bottom wall and a cylindrical wall. The cylindrical wall is of a diameter which is essentially the desired diameter for the finished can body, but is of a height which is substantially less than the desired final dimension. In addition, the thickness of the cylindrical wall is greater than that desired for the finished can body. The receptacle is then supported along its interior, and a hard metal ring having an inside diameter equal to the desired diameter of the completed can body is passed over the outside of the receptacle from the bottom wall along the cylindrical wall. This portion of the method, known as ironing, thins and stretches the metal of the cylindrical wall in the axial direction, thereby increasing the height of the receptacle. Upon the completion of ironing, the cylindrical wall is of the desired thickness and is of a height greater than the desired height for the finished can body.
In a second method, known as the "draw and redraw" method, a blank is formed into a cup-shaped receptacle, much as is done in the draw and iron method. However, the receptacle is formed having a diameter for the cylindrical wall that is somewhat greater than the desired diameter for the finished can body. The receptacle is then placed into tooling having a draw ring of the desired final diameter, and is drawn over the ring to reform the receptacle to the desired diameter, which also thins the cylindrical wall to the final desired thickness. The thinning and drawing lengthens the cylindrical wall in the axial direction to a length slightly greater than that desired for the completed can body. More than one redraw may take place.
Using either method, or combinations of those methods, further steps may be required or desirable, for example, providing a buckle-resistant shape to the bottom wall of the can body such as a central concave depression, or providing an inward necking of the upper end of the can body. In addition, it will be necessary to provide the body curl required for subsequent seaming of the body to the can end.
Can bodies formed by the methods described above are subject to a condition at the upper edge of the cylindrical wall which is known in the art as "earring". As the blank of metal is drawn into the cup-shaped receptacle and particularly as the receptacle is ironed or redrawn into the finished can body, considerable stretching of the metal stock occurs. However, this stretching is not entirely uniform throughout the blank, since the metal tends to stretch slightly more with the grain of the metal than across the grain. The result of such uneven "growth" of the metal is an uneven upper edge for the finished can body.
A can body exhibiting such earring is not desirable for seaming to the can end since the metal of the body curl may not tuck completely into the end curl on the can end at certain points along the can edge. In such a case, a complete seal may not be formed. Alternatively, certain portions of the body curl may tuck too far into the end curl, thereby distorting or even puncturing the material of the can end as seaming is completed.
To avoid these problems, it is necessary to carry out the additional step of trimming the upper end of the can body prior to formation of the body curl to provide a uniform upper edge. This eliminates the unevenness or earring, thereby ensuring proper sealing of the can end. However, the trimming operation results in waste of the can material. When multiplied by the very large number of cans that are manufactured, the amount of such material that is lost becomes substantial.
It is probably not possible to eliminate earring entirely, since some earring may result from local inconsistencies in the metal of specific blanks. However, if the degree of earring could be reduced, the amount of material which must be trimmed for each can body could then be decreased. In view of the large amount of material lost through the trimming operation, such savings would be significant.
What is needed, therefore, is a method and apparatus for forming can bodies which minimizes the amount of earring present on the finished bodies. Of course, to avoid creating other problems, such a method and apparatus should not result in any change in the dimensions of the finished can bodies, nor should such a method and apparatus require an increase in the amount of material required for each can body.